Method for quantifying oxygen stored in a vehicle emission control device

ABSTRACT

A method and system are provided for determining a present value representative of the quantity of oxygen that is stored in an emission control device receiving exhaust gas generated by an internal combustion engine, as well as a present value for the maximum capacity of the device to store a selected constituent gas of the exhaust gas. The resulting values are advantageously used for adaption of predetermined values for controlling device fill and purge times when operating in an open-loop control mode, whereby open-loop device operation is optimized to reflect changes in device operating conditions.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to emission control devices, positioned within an exhaust treatment system for a vehicle's internal combustion engine, which store oxygen present in the engine-generated exhaust gas during certain engine operating conditions and which release previously-stored oxygen during other engine operating conditions.

[0003] 2. Background Art

[0004] Generally, the operation of a vehicle's internal combustion engine produces engine exhaust that includes a variety of constituent gases, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NO_(x)). The rates at which the engine generates these constituent gases are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR. Moreover, such engines often generate increased levels of one or more constituent gases, such as NO_(x), when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio, for example, to achieve greater vehicle fuel economy.

[0005] In order to control vehicle tailpipe emissions, the prior art teaches vehicle exhaust treatment systems that employ one or more three-way catalysts, also referred to as emission control devices, in an exhaust passage to store and release select constituent gases, such as oxygen and NO_(x), depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NO_(x) when the exhaust gas is lean, and releases previously-stored NO_(x) when the exhaust gas is either stoichiometric or “rich” of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio.

[0006] Such systems often employ open-loop control of device storage and release times (also respectively known as device “fill” and “purge” times) with a view toward maximizing the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes “filled.” The timing of each purge event must be controlled so that the device does not otherwise exceed its NO_(x) storage capacity, because NO_(x) would then pass through the device and effect an increase in tailpipe NO_(x) emissions. The frequency of the purge is preferably controlled to avoid the purging of only partially filled devices, due to the fuel penalty associated with the purge event's enriched air-fuel mixture.

[0007] The prior art has recognized that the storage capacity of a given emission control device as to both oxygen and selected constituent gases is itself a function of many variables, including device temperature, device history, sulfation level, and the presence of any thermal damage to the device. Moreover, as the device approaches its maximum capacity, the prior art teaches that the incremental rate at which the device continues to store the selected constituent gas may begin to fall.

[0008] Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NO_(x)-storage capacity for its disclosed device which is significantly less than the actual NO_(x)-storage capacity of the device, to thereby provide the device with a perfect instantaneous NO_(x)-absorbing efficiency, that is, so that the device is able to store all engine-generated NO_(x) as long as the cumulative stored NO_(x) remains below this nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NO_(x) reach the device's nominal capacity. Unfortunately, however, the use of such a fixed device capacity necessarily requires a larger device, because this prior art approach relies upon a partial, e.g., fifty-percent NO_(x) fill in order to ensure retention of all engine-generated NO_(x).

SUMMARY OF THE INVENTION

[0009] It is an object of the invention to provide a method and system for determining a value representative of a quantity of oxygen stored in a vehicle emission control device, as may be useful for optimizing open-loop control of the filling of the device with a selected constituent gas of the engine-generated exhaust gas during lean-burn engine operation.

[0010] Under the invention, a method and system are provided for quantifying oxygen stored in an emission control device used to reduce emission of a selected constituent gas of engine-generated exhaust gas, by periodically generating an open-loop first and second fill time during which the device is respectively filled with the selected constituent gas to respective, sub-optimal capacity levels; and filling and purging the device based on the generated first and second fill times within a predetermined number of successive fill and purge cycles. Then, for each sub-optimal fill, an output signal is detected from an oxygen concentration sensor positioned within the flow of exhaust gas passing through the device, and a first and second actual purge time respectively corresponding to the first and second fill times are generated based on the detected output signal. A value representative of the quantity of oxygen stored in the device is then generated as a function of both the first and second fill times, and the first and second actual purge times.

[0011] Thus, the present invention provides a method and system which periodically determines the amount of oxygen being stored in the device, which can be used along with a value for a total device capacity to determine the actual capacity of the device to store the selected constituent gas, such as NO_(x). In this manner, vehicle tailpipe emissions can be advantageously minimized while simultaneously minimizing the fuel consumption associated with purging of the device.

[0012] The above object and other objects, features, and advantages of the invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 shows a schematic view of a lean-burn exhaust system in a four-cylinder engine accordance with the invention;

[0014]FIG. 2 shows a typical voltage vs. air-fuel ratio response for an oxygen sensor;

[0015]FIG. 3 is a representation of an exemplary lookup map used to store initial and learned values in accordance with a preferred embodiment of the invention;

[0016] FIGS. 4(a)-(d) show plots of associated responses of engine air-fuel ratio, oxygen sensor response, tailpipe CO during a short, medium and long purge time cycle, and the associated data capture window;

[0017]FIG. 5 shows an enlarged view of the response of the tailpipe oxygen sensor to the three levels of purge time shown in FIG. 4(c);

[0018]FIG. 6 shows a plot of normalized oxygen sensor saturation time t_(sat) as a function of purge time t_(P);

[0019]FIG. 7 shows a plot of normalized saturation time t_(sat) versus oxygen sensor peak voltage V_(P) when the peak voltage V_(P) is less than a reference voltage V_(ref);

[0020]FIG. 8 shows a plot of purge time t_(P) verse device fill time t_(F), including points for saturation and sub-optimal points A and B;

[0021]FIG. 9 is a flowchart showing overall system optimization for purge time t_(P) and fill time t_(F) in accordance with the invention;

[0022]FIG. 10 is a flowchart showing a saturation purge time optimization routine in accordance with the invention;

[0023]FIG. 11 is a flowchart showing a point B purge time determination routine in accordance with the invention;

[0024]FIG. 12 is a flowchart showing a point A purge time determination routine in accordance with the invention;

[0025]FIG. 13 is a flowchart showing a routine for determining a purge time value representative of oxygen stored in the device;

[0026]FIG. 14 is a flowchart showing a routine for determining a purge time value representative of NO_(x) stored in the device; and

[0027]FIG. 15 is a flowchart showing a normal purge time optimization routine in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] Referring to FIG. 1, a lean burn engine exhaust system 10 in accordance with the invention includes an engine 12 having a conventional exhaust manifold 14 coupled thereto, a first, upstream catalytic emission control device 16 receiving exhaust gas from the exhaust manifold 14, and a second, downstream catalytic emission control device 18 coupled to the upstream device 16 by an exhaust pipe 20. At least one fuel injector 22 delivers fuel to the engine 12 during operation thereof. While four such injectors are shown, this is not to be construed as limiting. A fuel injection controller in the form of a powertrain control module (PCM) 24 controls operation of the fuel injector(s) 22 in accordance with any conventional fuel control algorithm strategy such as proportional integral (PI) with jump and ramp, proportional integral differential (PID), or equivalent. PCM 24 includes a central processor unit (CPU) 26, and associated RAM 28 and ROM 30 memory.

[0029] A first oxygen sensor 32 is coupled to the exhaust manifold 14 and PCM 24 for controlling the air-fuel ratio of the engine exhaust during normal operation of engine 12. As discussed in more detail below, a closed-loop air-fuel ratio control is provided by connecting PCM 24 to a second oxygen sensor 34 coupled to the downstream device 18 for controlling air-fuel ratio and adapting various device purge parameters during operation of the engine. The second oxygen sensor 34 is preferably positioned in the exhaust output portion of the downstream device 18 so as to provide an accurate measurement of the air-fuel ratio exiting the device 18. A temperature sensor 42, preferably located at a mid-point within the downstream device 18, generates an output signal representative of the instantaneous temperature T of the device 18. In a constructed embodiment, the first and second oxygen sensors 32, 34 are “switching” heated exhaust gas oxygen (HEGO) sensors; however, the invention contemplates use of other suitable sensors for generating a signal representative of the oxygen concentration in the exhaust manifold 14 and exiting the downstream device 18, respectively, including but not limited to exhaust gas oxygen (EGO) type sensors, and linear-type sensors such as universal exhaust gas oxygen (UEGO) sensors. A typical voltage versus air-fuel ratio response for a HEGO sensor is shown in FIG. 2.

[0030] An alternative lean burn engine exhaust system could be employed utilizing a split exhaust design having two separate exhaust manifolds respectively coupled to an associated three-way catalyst. Individual exhaust manifold oxygen sensors would be provided for non-lean burn operation. In both the single and split exhaust design, air is input through an intake manifold 36 under control of a throttle 38.

[0031] During lean operation of the engine, at least one constituent gas of the engine generated exhaust, such as NO_(x), passes through the upstream device 16 and is stored in the downstream device 18. This portion of operation is referred to as the “fill time t_(P)” for the downstream device 18. As described in more detail below, device fill time t_(P) is initially controlled in an open-loop manner using predetermined nominal values for device purge time t_(P) and device fill time t_(F) stored in ROM 30. These predetermined nominal values are subsequently adapted to adjust the device fill and purge operation to changing device conditions during the life of the vehicle.

[0032] Oxygen sensor 32 is used for control of the engine air-fuel, especially during any stoichiometric operation, while oxygen sensor 34 is used for diagnosis of the downstream device 18, and determination of actual in-operation values of t_(P) for NO_(x) (t_(P) _(NOx) *), t_(P) for oxygen mass (t_(P) _(osc) *), and t_(F)*. Determination of these in-operation or “learned” values allows closed-loop adaption of the predetermined nominal values in accordance with the invention. These learned adaptive values are also compared to respective threshold values to allow the CPU to assess the degree of device deterioration. If deterioration is severe enough, a device regeneration cycle featuring, for example, a device desulfation event, is scheduled; or a warning is activated to indicate that the downstream device 18 requires servicing or replacement.

[0033]FIG. 9 provides a flowchart illustrating an exemplary device control process in accordance with the invention. As denoted at block 100, normal control is open-loop for both fill and purge cycles using the initial and/or learned values for fill and purge time. The nominal value for purge time t_(P)(i, j) is set to a fixed percentage of the nominal device saturation value t_(P) _(sat) , such as 80%.

[0034] As denoted at block 102, the values for t_(P) are periodically optimized in accordance with the optimization routine shown in FIG. 15 and described more fully below. This optimization process allows values for t_(F) to be adapted as well. A running counter is incremented at block 104 after every device event is performed to provide a mechanism for monitoring the length of time the engine has been in operation. A device event can be each individual fill or purge cycle, or a pair of corresponding fill and purge cycles.

[0035] At block 106, the control processor determines whether a predetermined number of events has occurred. The predetermined number is set to a large number so that the optimization routine of FIG. 15 is performed many times at a substantially steady state condition before a yes decision is generated at block 106. For example, the predetermined number can be between 1,000-10,000 events.

[0036] If the predetermined number of events has occurred, a saturation purge time optimization routine is performed as indicated at block 108. This routine is shown in FIG. 10, and as described in more detail below, produces a new value for t_(P) _(sat) *. After the new t_(P) _(sat) * value is stored, a first sub-optimal purge time t_(P) _(B) determination routine is performed at block 110. This routine is shown in FIG. 11, and as also described below, produces a new value for t_(P) _(B) *. After the new t_(P) _(B) * value is stored, a second sub-optimal purge time determination routine is performed at block 112. This routine is similar to that for first sub-optimal purge time and is described below in connection with FIG. 12. This routine likewise produces a new purge time value t_(P) _(A) *.

[0037] It is noted that the order of performing blocks 108-112 is not critical to the invention, and the sequence shown is only for illustrative purposes. However, performing all three routines within a small number of events of each other improves the reliability of the results.

[0038] After the new value for t_(P) _(A) * is stored, a new value for t_(P) _(osc) * is determined at block 114 as described below in context with FIG. 13. Then, as denoted at block 116, a new value for t_(P) _(NOx) * is determined as a function of t_(P) _(sat) * and t_(P) _(osc) *. This operation is described below in context with FIG. 14. Each of these values constitute learned values which optimize device performance to changing physical conditions during the life of the device.

[0039] A more detailed explanation of the invention will now be made in connection with FIGS. 3-8, and 10-15. In accordance with the invention, engine operating conditions are classified in a matrix of cells as a function of engine load (i) and engine speed (j). Nominal values for device fill time t_(F)(i, j) are provided on a cell by cell basis. More specifically, an initial device NO_(x) capacity NO_(x) _(cap) (i, j) is predetermined for a cell from a lookup map of such values as represented in FIG. 3. It is noted that all values, whether initial/nominal values, or learned adaptive values are preferably stored in similar i/j lookup table as shown in FIG. 3. Thus, the NO_(x) generation rate NO_(x) _(gen) (i, j) for a cell is determined from a corresponding lookup table. The nominal value for a cell's device fill time is then determined as follows: ${t_{F}\left( {i,j} \right)} = \frac{{NOx}_{cap}\left( {i,j} \right)}{{NOx}_{gen}\left( {i,j} \right)}$

[0040] These nominal values for t_(F)(i, j) are also stored in a corresponding lookup table.

[0041] As noted above, the nominal cell values for fill time are adapted to adjust for changes in device operating conditions during operation of the engine. Adaptive factors K(i, j) are learned during the closed-loop feedback control of the purge and fill times as described below in context with FIG. 15. These adaptive factors are stored in a corresponding lookup table and are used to adapt nominal values for both fill time t_(F) and purge time t_(P).

[0042] However, such adaptive factors are only selectively updated or applied as corrective measures if a certain level of reliability has been attained. More specifically, if PCM 24 determines that operation of the engine was confined to a single cell (i, j) or a small cluster of adjacent cells prior to a purge event, then reliability of the adaptive process is satisfied. Once reliability is satisfied, actual fill time t_(F)*(i, j) can be determined from closed-loop purge and fill control, and compared with the nominal fill time value t_(F)(i, j). When t_(F)*(i, j)<t_(F)(i, j)−ε, where ε is a predetermined tolerance, NO_(x) capacity NO_(x) _(cap) (i, j) of the cell (i, j) has deteriorated. Adaptive compensation is provided by choosing: ${K\left( {i,j} \right)} = \frac{p \times {{{t_{F}^{*}\left( {i,j} \right)} - {t_{F}\left( {i,j} \right)}}}}{t_{F}\left( {i,j} \right)}$

[0043] where p is a gain constant between 0 and 1, and the cell values for K(i, j) are initially set equal to 1.0. As each cell is compensated, the stored mapping of each adaptive factor is updated as the engine is subsequently operated in the corresponding cell. Otherwise, if operation was not confined to a single cell or adjacent cluster of cells, reliability has not been satisfied and PCM 24 will not consider any correspondingly generated adaptive values as being reliable. In this situation, PCM 24 will continue to use the nominal values, or apply only the most recent reliably generated adaptive factors.

[0044] During normal device control cycles, actual fill time t_(F)*(i, j) can be determined during transient operation, i.e., operation of the engine across different cells during a single cycle, by utilizing the known adaptive factor K(i, j), and corresponding nominal values for fill time t_(F) for each cell in which operation occurred. More specifically, the NO_(x) capacity remaining after operating in a particular cell for a period of time t(i, j) is given by:

NOx _(cap) _(—) _(avail) =K(i,j)NOx _(cap)(i,j)−NOx _(gen)(i,j)×t(i,j).

[0045] Since the engine operating point moves from cell to cell during transient operation of the engine, the NO_(x) capacity remaining after moving through several cells is given by: ${{NO}\quad x_{cap}} = {\sum\limits_{{i = 1},{j = 1}}^{n,m}{\left( {{{K\left( {i,j} \right)}{NO}\quad {x_{cap}\left( {i,j} \right)}} - {{K\left( {i,j} \right)}{NO}\quad {x_{gen}\left( {i,j} \right)} \times {t\left( {i,j} \right)}}} \right).}}$

[0046] When NO_(x) _(cap—avail) =q, where q represents a desired reserve NO_(x) capacity, a purge event is scheduled.

[0047] The total purge time t_(P)(i, j), in a given cell, is given by:

t _(P)(i,j)=t _(P) _(NOx) (i,j)×K+t _(P) _(osc) *(i,j),

[0048] where t_(P) _(NOx) is either the mass of fuel or amount of time required to purge the stored NO_(x), and t_(P) _(osc) * is either the actual mass of fuel or the amount of time required to purge oxygen stored in the device. The following description explains how these two components of the total purge time are determined. Nominal values for t_(P) _(NOx) are stored in a corresponding lookup table.

[0049] The mass of oxygen stored during a fill event is given by OSC (gm). In many known catalytic emission control devices, oxygen is typically stored as one of the oxides of cerium as a function of engine speed and load. Oxygen can also be stored as a precious metal oxide. The stored-oxygen purge time t_(P) _(osc) (i, j) (sec) for a given cell (i, j) is determined as described below in connection with FIG. 13. Nominal values for total purge time t_(P) and stored-oxygen purge time t_(P) _(osc) are derived from a lookup table.

[0050] A description of the in-use purge and fill time optimization routines of the invention will now be made in connection with FIGS. 4-8 and 10-15. For purposes of understanding the invention, system responses exhibited during three different lengths of the device purge time are shown in FIGS. 4(a)-(c). More specifically, FIG. 4(a) shows the relationship of lean fill time t_(F) and rich purge time t_(P) for three different purge times of short (1), medium (2), and long (3) duration. The corresponding response of oxygen sensor 34 is shown in FIG. 4(b) for the same three purge times. As can be seen, a small purge time (1) produces a very small oxygen sensor response as a result of the device not being fully purged of NO_(x) and still having a considerable amount of residual NO_(x) stored therein. For a short purge time, the peak sensor voltage will not reach a reference voltage V_(ref). For a moderate, or optimum, purge time (2), the oxygen sensor response equals the reference voltage V_(ref) indicating that the device has been adequately purged. For a long purge time (3), the oxygen sensor peak voltage V_(lain P) exceeds the reference voltage V_(ref), indicating that the device has been over purged, thereby undesirably generating excessive tailpipe CO as shown in FIG. 4(c).

[0051] Referring more specifically to FIGS. 4(c) and 5, a data capture window (shown in FIG. 4(d)) for sampling the output voltage of the downstream oxygen sensor 34 is timed relative to the purge event such that for short purge time (1), very little CO passes through the device into the tailpipe and thus produces a small output response. Thus, the invention uses the peak voltage level and its duration above a threshold voltage of the downstream oxygen sensor 34 as an indicator of the quantity of NO_(x) still stored in the downstream device 18. FIG. 5 shows an enlarged view of the response of the downstream oxygen sensor 34 to the three levels of purge time shown in FIG. 4(c). FIG. 5 illustrates a saturation time Δt₂₁ for a situation where the V>V_(ref).

[0052]FIG. 6 shows an extrapolated relationship between a normalized oxygen sensor saturation time t_(sat) and the purge time t_(P). The plot show three regions for a given fill time t_(F) and operative state of the downstream device 18, the first region being defined by V=0 (no response from the HEGO sensor 34); the second region being defined by V<V_(ref) (lean response from HEGO sensor 34); and the third region being defined by V>=V_(ref) (a rich response from HEGO sensor 34). Purge time t_(P) _(ref) results in a saturation time t_(sat) _(ref) , which is the minimum normalized time resolution for the system (as normalized by t_(sat) desired). A purge time t_(P) _(min) or less results in a zero value for saturation time t_(sat). A purge time of t_(p) _(sat—desired) results in a saturation time t_(sat) equal to one.

[0053] Situations where t_(sat)>1 are thus indicative that the purge time t_(P) should be decreased, while situations where t_(sat)<1 are indicative that the purge time t_(P) should be increased. This forms the basis by which the invention provides a closed-loop process for optimizing or correcting purge times for a given fill time. For t_(sat)>t_(sat) _(ref) , a metric for t_(sat) involves direct measurement of the time that the oxygen sensor voltage exceeds V_(ref) by PCM 24. For t_(sat)<t_(sat) _(ref) , the PCM 24 uses the relationship shown in FIG. 7 to provide a smooth continuation to the metric of FIG. 5 by linearly extrapolating the saturation time from t_(sat)=t_(sat) _(ref) to t_(sat)=0, making t_(sat) proportional to the peak sensor voltage V_(P).

[0054]FIG. 8 shows the nominal relationship between purge time t_(P) and fill time t_(F) for a given operating condition of the engine and a given condition of the downstream device 18. This relationship holds for an approximately constant saturation t_(sat). The purge time t_(P) monotonically increases with the fill time t_(F) but reaches saturation when the capacity of the device 18 is equaled or exceeded. A purge time t_(P) which simultaneously maximizes the storage of NO_(x) in the device 18, minimizes CO tailpipe emissions during purging, and optimizes the fill time t_(F) is designated as optimized purge time t_(PT). The optimized purge time t_(PT) corresponds to an optimized fill time t_(FT). Normal purge-time optimization is periodically performed in accordance with the routine of FIG. 15 and, as described below, generates the learned adaptive values K(i, j) for each cell. These adaptive values are subsequently used during normal open-loop fill and purge control as noted above.

[0055] Referring now to FIG. 15, the optimization subroutine for purge time t_(P) executed at block 102 of FIG. 9 is illustrated in further detail. As noted previously, this subroutine optimizes the air-fuel ratio rich purge spike for a given value of fill time t_(F). First, as denoted at block 700, initial values for fill time t_(F) and purge time t_(P) are retrieved from corresponding lookup tables. These values are either nominal values or previously learned values t_(F)* and t_(P)*, depending on the current operating cell or state of the vehicle engine. The downstream device 18 is then filled and purged at blocks 702 and 704 according to the retrieved values.

[0056] At block 706, the PCM determines wether steady state speed/load conditions were present during the fill/purge events of blocks 702 and 704. If steady state conditions were not present, the routine exits the optimization routine. However, if steady state conditions were present, the routine samples the downstream oxygen sensor output during the data sample window as indicated at block 708. As noted previously, the window is timed relative to the device purge event so as to capture the change in the downstream oxygen sensor output as shown in FIG. 4(b).

[0057] The peak voltage V_(P) of the sensor is then determined and compared to V_(ref) at block 710. If the peak sensor voltage V_(P) is greater than the reference voltage V_(ref), the incremental time Δt₂₁ spent above the reference voltage V_(ref) is measured at block 714, and a conversion is made to a saturation time t_(sat) proportional to Δt₂₁ at block 716. On the other hand, if the peak sensor voltage V_(P) is less than the reference voltage V_(ref), the saturation time t_(sat) is determined at block 712 from the linearly extrapolated function where t_(sat) is proportional to V_(P). Using this metric provides a smooth transition from V_(P)<V_(ref) to V_(P)=V_(ref).

[0058] A saturation time error t_(sat) _(error) for the actual saturation time t_(sat) relative to an optimal or desired value t_(sat) _(desired) (the target value for the metric which optimizes the system in terms of minimum CO, HC and NO_(x) emissions, which may preferably vary as a function of engine operating parameters such as engine speed, engine load, and device temperature) is calculated by subtracting the actual device saturation time t_(sat) from the desired value as shown in block 718. The saturation time error t_(sat) _(error) is then normalized as shown in block 720, and used as an input to a feedback controller, such as a PID (proportional differential integral) algorithm at block 722. The output of the PID controller generates a multiplicative correction factor PURGE_MUL which is then stored as a new adaptive value K(i, j) in the associated lookup cell. The PCM uses the adaptive value K as indicated at block 724 to adapt the purge time t_(P) in subsequent open-loop cycles to provide optimized “learned” purge times t_(P)*(i, j) for given fill times t_(F)(i, j). In addition, these same adaptive values K are used to adapt the stored fill times t_(F)(i, j) to generate “learned” fill time values t_(F)*(i, j) which correspond to the adapted purge time values. These learned values are stored in a corresponding lookup table. Alternatively, instead of changing the purge time t_(P), the strength of the purge, i.e., the air-fuel ratio of the air-fuel mixture employed during the purge event (as shown in FIG. 4), can be adjusted in a similar manner.

[0059] In further accordance with the invention, learned values for actual NO_(x) purge time t_(P) _(NOx) * are obtained after learning both the values for total purge time t_(P)* for a given cell, and the purge time t_(P)* related to the quantity of oxygen stored in the downstream device 18. More specifically, to determine t_(P) _(osc) *, a current saturation value t_(sat)*, i.e., the value corresponding to point S on the “flat” portion of the purge time t_(P) versus fill time t_(F) function response of FIG. 8, is determined in accordance with the routine shown in FIG. 10. As denoted at block 200, initial values for a fill time t_(F) and a purge time t_(P) are retrieved from a lookup table. These initial values are selected to ensure a longer than normal fill and purge time. The downstream device 18 is then filled and purged at blocks 202 and 204 in accordance with the retrieved initial values.

[0060] At block 206, the processor determines whether steady state speed/load conditions were present during the fill/purge events of blocks 202 and 204. If steady state conditions were not present, the routine simply returns to normal device control. However, if steady state conditions were present, the routine samples the downstream oxygen sensor output during the data sample window as indicated at block 208.

[0061] The peak voltage V_(P) of the sensor is then determined and compared to a reference voltage V_(ref) at block 210. If V_(P)>V_(ref), the incremental time Δt₁₂ spent above V_(ref) is measured at block 214, and a conversion is made to a t_(sat) proportional to Δt₂₁ at block 216. On the other hand, if V_(P)<V_(ref), the saturation time t_(sat) is determined at block 212 from the linearly-extrapolated function where t_(sat) is proportional to V_(P). Using this metric provides a smooth transition from V_(P)<V_(ref) to V_(P)=V_(ref).

[0062] An error in t_(sat) relative to the desired value t_(sat)=t_(sat) _(desired) is calculated and made equal to t_(sat) _(error) at block 218. The error is then normalized as shown in block 220, and used as an input to a feedback controller, such as a PID (proportional differential integral) algorithm at block 222. The output of the PID controller generates a multiplicative correction factor PURGE_MUL which is used at block 224 to adapt the purge time t_(P) _(sat) * in subsequent saturation determination cycles. At block 226, it is determined whether |t_(P) _(sat) *−t_(P) _(sat) |<ε, where ε is an allowable tolerance. If not, tPsat is set to tPsat* and another saturation purge is scheduled, and the downstream device 18 is filled at block 202. If |t_(P) _(sat) *−t_(P) _(sat) |<ε, the learned value for t_(P) _(sat) * is stored in a corresponding lookup table at block 228.

[0063] As shown in FIGS. 11-13, actual values for purge time responsive to stored oxygen (t_(P) _(osc) *) are also obtained through closed-loop control of the purge and fill times. In accordance with the invention, t_(P) _(osc) * is determined using two sub-optimum fill and purge times corresponding to points A and B in FIG. 8. These points are less than the optimum fill time and are selected to coincide with the proportionally linear portion of the response curve.

[0064] More specifically, actual purge times t_(P) _(B) * and t_(P) _(A) * for point B and point A, respectively, are determined using separate routines similar to that for saturation purge time determination, but using stored fill and purge time values (t_(F) and t_(P)) corresponding to points A and B. The specific steps are shown as blocks 300-328 in FIG. 11 for point B, and blocks 400-428 in FIG. 12 for point A.

[0065] As shown in FIG. 13, the updating process for purge time corresponding to stored oxygen (t_(P) _(osc) *) is obtained by initially setting cell values i and j to 1 at block 500, and retrieving the stored values for t_(P) _(A) *(i, j), t_(F) _(A) , t_(P) _(B) * (i, j) and t_(F) _(B) at block 502.

[0066] An updated value is determined at block 504 in accordance with the following: ${t_{P_{osc}}^{*}\left( {i,j} \right)} = {\frac{{{t_{P_{B}}^{*}\left( {i,j} \right)} \times t_{F_{B}}} - {{t_{P_{A}}^{*}\left( {i,j} \right)} \times t_{F_{A}}}}{t_{F_{A}} - t_{F_{B}}}.}$

[0067] The values for t_(P) _(osc) * are stored in a corresponding lookup table as indicated at block 506. As denoted by blocks 508-516 the entire table is stepped through to update all cell values.

[0068] As shown in FIG. 14, the current values of t_(P) _(osc) *(i, j), together with the current values of t_(P)*(i, j), are used to update the values for NO_(x) purge time t_(P) _(NOx) *(i, j). Initially, i and j are set to 1 at block 602, and the values for t_(P) _(osc) *(i, j) and t_(P)*(i, j) are retrieved from memory at block 604. At block 606 the current value for t_(P) _(NOx) *(i, j) is determined as t_(P) _(NOx) *(i, j)=t_(P)*(i, j)−t_(P) _(osc) *(i, j). The updated value for t_(P) _(NOx) *(i, j) is then stored in the lookup table. Each cell of the lookup table is updated in this manner as denoted in blocks 610-618.

[0069] In further accordance with the invention, current values of t_(P) _(NOx) *(i, j) can be compared with the initial values t_(P) _(NOx) (i, j) to assess the degree of device deterioration. That is, if t_(P) _(NOx) *(i, j)<t_(P) _(NOx) (i, j)−α, where α is a predefined constant, then the NO_(x) storage capacity of the downstream device 18 has deteriorated, HC and CO emissions have increased, and a desulfation or “de ” event is scheduled. In addition, t_(P) _(osc) *(i, j) can be compared to t_(P) _(osc) (i, j), and the driver notified via a warning circuit that servicing is required if there is a difference greater than a predetermined tolerance value.

[0070] Thus, the invention advantageously provides a method and system which accurately discriminates the condition or “health” of the downstream device 18 to provide real-time feedback loop control of device fill time, purge time, and strength of purge, during engine operation in a vehicle. The invention therefore allows a catalytic emission control device to be continuously operated at optimum efficiency.

[0071] While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. 

What is claimed:
 1. A method for quantifying oxygen stored in an emission control device that receives exhaust gas generated by an internal combustion engine, wherein the device is filled with a constituent gas of the exhaust gas during a first engine operating condition and is purged of constituent gas during a second engine operating condition, the device having an optimal capacity level for storing the constituent gas, the method comprising: periodically generating an open-loop first and second fill time, wherein the first and second fill times are different values established to cause the device to be filled to a first and second sub-optimal capacity level, respectively; filling and purging the device based on the generated first and second fill times within a predetermined number of successive fill and purge cycles; and for each sub-optimal fill, detecting an oxygen concentration within the flow of exhaust gas passing through the device; generating a first and second actual purge time respectively corresponding to the first and second fill times based on the detected output signal; and determining a value representative of the quantity of oxygen stored in the device as a function of both the first and second fill times, and the first and second actual purge times.
 2. The method of claim 1, further comprising determining a value representative of current total device capacity within a predetermined number of fill and purge cycles of performing the first and second sub-optimal fill cycles, and updating at least one purge time value as a function of the quantity of oxygen value and the total capacity value.
 3. The method of claim 2, wherein determining a first value representative of current total device capacity comprises: filling the device to saturation; purging the device; detecting an output signal from an oxygen concentration sensor positioned downstream from the device; generating an error signal as a function of the detected output signal and a predetermined reference value; and determining an actual purge time necessary to purge the saturated device of substantially all constituent gas.
 4. The method of claim 1, wherein detecting the oxygen concentration of the exhaust gas flowing through the device includes positioning a first oxygen sensor downstream of the device.
 5. The method of claim 1, wherein the first and second purge times are generated only if steady engine speed and load conditions are detected.
 6. A system for quantifying oxygen stored in an emission control device that receives exhaust gas generated by an internal combustion engine, wherein the device is filled with a constituent gas of the exhaust gas during a first engine operating condition and is purged of constituent gas during a second engine operating condition, the system comprising: an oxygen sensor generating an output signal representative of a concentration of oxygen present in the exhaust flowing through the device during a sampling period; and a control module including a microprocessor arranged to operate in a normal open-loop mode of operation by determining fill times and purge times based on engine operating conditions and corresponding values stored in a memory, wherein the control module is further arranged to periodically generate a first and second sub-optimal fill time, and determine corresponding first and second actual purge times required for purging constituent gas stored in the device during the first and second fill times, and to determine a value representative of the quantity of oxygen stored in the device as a function of both the first and second fill times, and the first and second actual purge times.
 7. The system of claim 6, wherein the oxygen sensor is positioned downstream from the device.
 8. The system of claim 6, wherein the oxygen sensor comprises a switching-type sensor.
 9. The system of claim 6, wherein the processor is further arranged to determine a value representative of total device capacity by filling and purging the device to saturation, update at least one purge time value as a function of the quantity of oxygen value and the total capacity value. 